1. Field of the Invention
The present invention relates to dielectric materials of excellent quality that have a relatively high permittivity (hereinafter referred to as .di-elect cons..sub.r) in a high frequency region, a small absolute value of the temperature coefficient of resonance frequency (hereinafter referred to as .tau..sub.f of f.sub.0), and a high unloaded quality factor (hereinafter referred to as Q.sub.u). The dielectric materials of the invention can be used in applications such as multilayer circuit boards, and, in particular, high-frequency resonators and filters. This invention further relates to processes for producing the dielectric materials.
2. Discussion of the Related Art
Dielectric materials based on BaO--ZnO--Ta.sub.2 O.sub.5 or BaO--MgO--Ta.sub.2 O.sub.5 are frequently used in high-frequency applications. Such dielectric materials for use in high-frequency applications must satisfy the following requirements:
(i) a high .di-elect cons..sub.r ; PA1 (ii) a small absolute value of .tau..sub.f ; and PA1 (iii) a high Q.sub.u in a high-frequency region.
The dielectric materials based on BaO--ZnO--Ta.sub.2 O.sub.5 or BaO--MgO--Ta.sub.2 O.sub.5 are oxides represented by the empirical formula Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3 or Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, respectively, that have a perovskite crystal structure. These oxides are generally referred to as BZT and BMT for short. Although BZT and BMT materials are excellent dielectric materials having a high Q.sub.u, there is a need for a dielectric material having an even higher Q.sub.u because these dielectric materials are increasingly used in a higher-frequency region, from the microwave to the sub-millimeter wavelength region. In addition, these dielectric materials generally are produced by complex and industrially undesirable processes, which include sintering at high temperatures exceeding 1,600.degree. C., sintering for a prolonged period, or sintering by ultrahigh-rate heating (see, for example, JP-B-6-25023, JP-A-4-224161, and JP-B-3-51242). (Here, "JP-B" and "JP-A" refer to an "examined Japanese patent publication" and an "unexamined published Japanese patent application," respectively.) Moreover, some trials of adding additives to these dielectric materials in order to improve sintering properties (see, for example, JP-B-1-18523 and JP-B-3-34164).